Injection molded solder (IMS) is a solder process technology applied primarily for silicon wafer bumping, where it is referred to as C4NP. Interconnect requirements continue to change, moving toward ever finer bump sizes and pitches. Especially as three-dimensional (3D) packaging grows, there exists a need for solder interconnects to extend down toward the micro-scale, with bump pitches at 50 microns or less. At such reduced sizes and pitches, standard glass molds used for IMS no longer suffice because of several problems.
For example, in existing approaches, with standard hemispherical cavities, the solder volume after reflow may be located anywhere around the perimeter of the cavity, which produces non-uniformities of location that can adversely affect transfer yields. As seen in other existing approaches, this is due to solder volume centers not aligning with wetting pad centers.
Additionally, as in some existing approaches, decreasing the diameter of the cavity while increasing the depth, which is required to maintain a given volume for a desired pitch, may help keep the reflowed solder centered but will not allow it to protrude from the mold surface sufficiently to assure wetting on the pads.
As such, a new microcavity structure and process for making such is needed to effectively allow IMS to be used for the anticipated micro bump applications required for 3D packaging.